The Operating System

Operating Systems (F)

• What you need to know about

– operating system as a program;– directory/folder

Operating Systems (G)

• What you need to know about

– interactive system;– real-time systems;– types of file: data files and programs.

Operating Systems (C)

• What you need to know about– HCI for the user.– file management, memory management,

error reporting– interactive systems with background job

capability;– device drivers– hierarchical filing system; – sequential and random/direct access

The Operating System

• The operating system (OS) is the most important program used by the computer.– The OS in most modern computers is stored on

hard disc and is loaded into RAM when the computer starts up.

– Some computers store their OS in ROM (e.g. some palmtop computers) ROM operating systems load very quickly (quicker than RAM OS) and they cannot be erased by accident.

The Operating System

– The operating system controls and monitors the operation of the computer system.

– The filing system is part of the OS and it controls the loading and saving of programs and data from backing storage.

Current Operating Systems

• Examples of operating systems which are disc based

– Windows XP for the PC– System X for Apple Macintosh– Linux for large fileservers and networks.

Current Operating Systems

• Examples of operating systems which are ROM based

– JavaOS for Network Computers (NCs)– PalmOS or Windows Mobile for palm top

and handheld devices (computers, phones, MP3 players etc.).

Standard Functions of the OS.

• User interface– the OS provides the means of

communicating with the computer system (Icons, commands etc. are all provided by the operating system).

• Manages memory– The way that memory is allocated to

programs is controlled by the OS. How much RAM each program gets.

Memory Management

• There is a limited amount of RAM to store programs in e.g. 1024mb

• If the OS takes up 512mb that leaves 512mb for storing any other program such Internet Explorer and Microsoft Word.

Memory Management

• As there is a limited amount of RAM as you open more and more programs the RAM allocated to each gets less and less.

• The less RAM a program has the slower it will run. The more RAM a program has the quicker it will run.

Memory Management

• RAM is used to store the parts the program which are going to be used next. These have loaded from hard disc.

• If there is little room in RAM to store these instruction they have to be loaded from hard disc which is a much slower process

Memory Management

The hard disk stores all the programs on a computer

When you open a program as much aspossible will be loaded in to RAM

The more of the program that is in RAM theQuicker it can run

As you load more and more programs in to RAM the slower each will run. Until they have so little RAM that they run really, really slowly

RAMThe Operating System

Internet Explorer

Microsoft Word

iTunes

Photoshop

Movie MakerHalo 3

Microsoft WordInternet Explorer

Creative Labs

Symphony

Standard Functions of the OS.

• Controls input/output– The access to all devices attached to the

computer is controlled by the OS. For example, the OS can stop two programs trying to use the same device at the same time.

Standard Functions of the OS.

• Filing System– The filing system manages backing storage– The access to all types of backing storage

devices is controlled by a special part of the operating system called the filing system.

– The filing system will prevent two programs trying to access the same files at the same time.

• Error Reporting– When an error occurs the operating system

provides information to the user about what has happened and, sometimes, how it can be fixed.

Resource allocation

• When a computer is running the demands on the systems resources (i.e. memory, input/output devices, backing storage etc.) may be greater than the resources available. To deal with this a resource allocation system is built into the OS.

Resource allocation

• Various techniques are used to prevent two programs demanding the same resource at the same time (e.g. two programs can't both send files to the printer at the same time).

• The resource allocation system also ensures that all programs running on the system receive as much processing time as possible.

OS - Different modes of processing

• Interactive• Accepting input from a human. • Interactive computer systems are programs that

allow users to enter data or commands. • Most popular programs, such as word processors

and spreadsheet applications, are interactive. • In interactive processing, the application responds

to commands as quickly as it can once they have been entered.

• Sometimes there is a delay while the processor finishes another task.

OS - Different modes of processing

• Real Time• Real-time operating systems are systems

that respond to input immediately.• Automatic teller machines for banks are an

example of real time processing. • Real time systems are used for tasks such

as navigation, in which the computer must react to a steady flow of new information without interruption.

OS - Different modes of processing

• Interactive systems with background job capability

• Some operating systems allow a background process to occur at the same time as a foreground interactive process.

• The foreground process is the one that accepts input from the keyboard, mouse, or other input device. Background processes cannot accept interactive input from a user, but they can access data stored on a disk and write data to the video display unit.

• For example, some word processors print files in the background, enabling you to continue editing while files are being printed.

Data files and Programs

• A program can be an application like a database or word processing program

• A data files is something that is created using an application program like a new word processing document

• You cannot use the data file unless the application program that created it is also there

• All computers can read simple text files or ASCII files, that’s why all computers can read emails no matter what application is used to compose the email

Hierarchical filing system

• A filing system in which directories have files and subdirectories beneath them

• A hierarchical filing system is one that uses directories to organise files into a tree structure.

• Directories (folders) can store more folders and/or files

Hierarchical filing system

C:/ D:/

Computer

Drives

Folders

Files

A Hierarchical Filing Structure

D:/

Types of Access

• Random Access (also called Direct Access) devices retrieve the data you want straight away e.g. CD-ROM drive, Hard disc drive, floppy disc drive.

• Sequential (also called Serial) devices have to move through all the data, record after record, to find the data you need e.g. Cassette tape, magnetic tape.

Types of access

• Sequential Access– To go from file A to file Z in a sequential-

access system, you must pass through all intervening files.

– Sequential access is sometimes called serial access.

Types of access

• Random Access– Refers to the ability to access data at

random. In a random-access system, you can jump directly to file Z. Disks are random access media, whereas tapes are sequential access media.

Device Drivers

• Computer operating systems are written by software companies e.g. Apple, Microsoft etc.

• It is impossible for these companies to know about every single item of hardware that has been or might be developed for their operating system.

• To get round this problem the manufacturer of an item of hardware writes a small program called a “device driver” for their device.

Device Drivers

• A device driver program includes instructions for the operating system about how to communicate with the hardware device.

• A printer driver takes the codes used in the document and translates them into code the printer understands